Suspension system in the car

Suspension is the system of tires, tire air, springs, shock absorbers and linkages that connects a vehicle to its wheels and allows relative motion between the two. Suspension systems must support both road holding handling and ride quality, which are at odds with each other. The tuning of suspensions involves finding the right compromise. It is important for the suspension to keep the road wheel in contact with the road surface as much as possible, because all the road or ground forces acting on the vehicle do so through the contact patches of the tires. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and rear suspension of a car may be different.

start

• Prepare for ASE Suspension and Steering (A4) certification test content area “B”

(Suspension System Diagnosis and Repair).

• List various types of suspensions and their

• Describe how suspension components

function to allow wheel movement up and

down and provide for turning.

forces.

After studying Chapter 85, the reader should

be able to:

OBJECTIVES:

air spring • anti-dive • anti-squat

ball joints • bulkhead • bump stop

center bolt • coil springs • composite leaf spring • control arms • cradle

full frame • gVW • hooke’s law

independent suspension • insulators

kingpin • ladder frame • lateral links

leaf springs • load-carrying ball joint

KEY TERMS:

mono leaf • non-load-carrying ball joint

perimeter frame • platforms • rebound clips

shackles • shock absorbers • space frame • spring pocket

• spring rate • springs • sprung weight • stabilizer bars • steering knuckles • stress riser • strut rod • struts • stub- type frame

torsion bar

unit-body • unsprung weight

wheel rate

KEY TERMS:

Street-driven cars and trucks use a suspension system to

keep the tires on the road and to provide acceptable riding

comfort.

A vehicle with a solid suspension, or no suspension, would

bounce off the ground when the tires hit a bump.

If the tires are off the ground, even for a fraction of a second, loss of control is possible.

FRAME CONSTRUCTION

Frame construction consists of channel-shaped steel beams

welded and/or fastened together The frame supports all the

“running gear”, including the engine, transmission, rear axle

assembly (if rear-wheel drive), and all suspension components.

Referred to as full frame, it is so complete that most vehicles can

usually be driven without the body Most trucks and larger

rear-wheel-drive cars use a full frame.

Ladder Frame A common name for a type of perimeter frame

where the transverse (lateral) connecting members are straight

across is ladder frame When viewed with the body removed, the

frame resembles a ladder Most pickup trucks are constructed with

a ladder-type frame See Figures 85–1 and 85–2

Figure 85–1 A typical truck frame is an excellent example of a ladder-type frame The two side = members are connected by a crossmember.

Figure 85–2 Rubber cushions used in body or frame construction isolate noise and vibration from traveling to the passenger compartment.

Perimeter Frame

A perimeter frame consists

of welded or riveted frame

members around the entire

perimeter of the body

This means frame members

provide support underneath

the sides as well as for the

suspension and suspension

components.

Figure 85–3 (a) Separate body and frame

construction; (b) unitized construction: the

small frame members are for support of the

engine and suspension components Many

vehicles attach the suspension components

directly to the reinforced sections of the body

and do not require the rear frame section.

Stub-Type Frames

A stub-type frame is a partial

frame often used on unit-body

vehicles to support power train

and suspension components

It is also called a cradle on many

front-wheel-drive vehicles

(a)

(b)

Unit-Body Construction Unit-body (called unibody) combines

the body with the structure of the frame The body supports the

engine and drive line components, as well as the suspension and

steering components The body is composed of many individual

stamped-steel panels welded together.

The strength of this type of construction lies in the shape of the

assembly The typical vehicle uses 300 separate, different stamped steel panels that are spot-welded to form a vehicle’s body

See Figure 85–4.

NOTE: A typical vehicle contains about 10,000 individual parts.

NOTE: A typical vehicle contains about 10,000 individual parts.

Figure 85–4 Welded metal sections create a platform that combines the body with the frame

using unit-body construction.

Space Frame Construction Formed sheet steel used to construct a

framework for the entire vehicle is called space frame The vehicle

is drivable without the body, which uses plastic or steel panels to

cover the steel framework.

What Does GVM Mean?

GVW, gross vehicle weight is the weight of the vehicle plus the weight of

all passengers the vehicle is designed to carry (150 lb [68 kg] each), plus the maximum allowable payload or luggage load

Curb weight is the weight of a vehicle when wet, meaning with a full tank of fuel and all fluids filled, but without passengers or cargo (luggage) Model weight is the weight of a vehicle wet and with passengers

The GVW is found stamped on a plate fastened to the doorjamb of most

vehicles A high GVW rating does not mean that the vehicle itself weighs a lot more than other vehicles For example, a light truck with a GVW of

6,000 lbs (2,700 kg) will not ride like an old 6,000-lb luxury car In fact, a

high GVW rating usually requires stiff springs to support the payload; these stiff springs result in a harsh ride

Often techs are asked to correct a harsh-riding truck that has a high GVW rating The tech can only check that everything in the suspension is

satisfactory and then try to convince the owner that a harsher-than-normal ride is the result of a higher GVW rating.

The platform of any vehicle is the basic size and shape Various

vehicles of different makes can share the same platform, and

therefore many of the same drive train (engine, transmission, and

final drive components) and suspension and steering components.

Other components of vehicle platform design that affect hand-ling

and ride are the track and wheelbase of the vehicle.

A platform of a unit-body vehicle includes all major sheet-metal

components that form the load-bearing structure of the vehicle,

including the front suspension and engine-supporting sections.

The area separating the engine compartment from the passenger

compartment is called the bulkhead The height and location of

the bulkhead panel determines the shape of the rest of the vehicle.

Track of a vehicle is distance between the wheels, as viewed from the front or rear.

A wide-track vehicle is a vehicle with a wide wheel stance; this

increases the stability of the vehicle, especially when cornering.

Hollander Interchange Manual

Most salvage businesses that deal with wrecked vehicles use a reference book called the Hollander Interchange Manual In this yearly publication, every vehicle part is given

a number If a part from another vehicle has the same Hollander number, then the parts are interchangeable.

A vehicle with a long wheelbase tends to ride smoother than a

vehicle with a short one.

Wheelbase of a vehicle is distance between the center of the front wheel and the center of the rear wheel, as viewed from the side.

UNSPRUNG WEIGHT

A suspension system has allow the wheels to move up and down

quickly over bumps and dips without affecting the entire weight of

the car or truck The lighter the total weight of the components, the

better the handling and ride This is called unsprung weight.

The idea of very light weight resulted in magnesium wheels for

racing cars, very light yet strong Aftermarket wheels that

resemble racing wheels are often referred to as mag wheels

Unsprung weight should be kept as low as possible.

Sprung weight is the term used to identify the weight of the car or

truck that does not move up and down and is supported or sprung

by the suspension.

TYPES OF SUSPENSIONS

Early suspension systems on

old horse wagons, buggies,

and older vehicles used a solid

axle for front and rear wheels

Figure 85–5 Solid I-beam axle with leaf springs

Figure 85–6 When one wheel hits a bump or drops into a hole, both left and right wheels are moved Because both wheels are affected, the ride is often harsh and feels stiff.

If one wheel hit a bump, the other wheel was affected, as shown above.

Most vehicles today use a separate control-arm-type of suspension for each front wheel, which allows for movement of one front

wheel without affecting the other front wheel This type of front

suspension is called independent suspension

Figure 85–7 A typical independent front suspension used on a rear-wheel-drive vehicle Each

wheel can hit a bump or hole in the road independently without affecting the opposite wheel

Springs A suspension spring serves two purposes It acts as a

buffer between the suspension and frame to absorb vertical wheel

and suspension movement without passing it on to the frame Each spring transfers part of the vehicle weight to the suspension

component it rests on, which transfers it to the wheels.

All springs give way to absorb the vertical force of the moving

wheel during jounce, then release that force during rebound as they return to their original shape and position Leaf springs flatten, coil and air springs compress, and torsion bars twist

Spring Materials Most are made of a tempered steel alloy known

as spring steel, usually chrome silicon or chrome-vanadium alloy Tempering is controlled heating and cooling metal to increase the

ability of the metal to return to, or spring back to, its original shape after being twisted or compressed.

HOOKE’S LAW

All suspensions use springs with a common characteristic

described Robert Hooke (1635–1703) An English physicist, he

discovered force the characteristics of springs

Figure 85–8 This spring was depressed 4 inches due to a weight of 2,000 Ib This means that

Deflection (movement or deformation) of a spring

is directly proportional

to the applied force.

Hooke’s Law

Coil springs are made of special

round spring steel wrapped in a

helix shape

1 Coil diameter

2 Number of coils

3 Height of spring

4 Diameter of the steel coil

that forms the spring

Figure 85–9 The spring rate of a coil spring is

determined by the diameter of the spring and

diameter of the steel used in its construction

plus number of coils and free length (height)

COIL SPRINGS

Characteristics of a coil spring

(strength, etc.) depend on:

When a coil spring (for example) is depressed 1 in., it pushes back with a certain force (in pounds), such as 400 pounds If the spring

is depressed another inch, force exerted by the spring is increased

by another 400 pounds.

The spring rate (K) for

coil springs is expressed

by the formula at right.

The spring rate or force

constant for this spring is

“400 lb per inch,” usually

symbolized by the letter K.

Since the force constant is the force per unit of displacement

(movement), it is a measure of the stiffness of the spring The

higher the spring rate (K), the stiffer the spring.

The larger the diameter of the steel, the “stiffer” the spring.

The shorter the height of the spring, the stiffer the spring.

The fewer the coils, the stiffer the spring.

Figure 85–10

Coil spring ends

are shaped to fit

the needs of a

variety of

suspension

designs.

Springs are designed to provide desired ride and handling and

come in a variety of spring ends, as shown here:

Spring Rate called deflection rate,

is the weight in pounds it takes to

compress the spring 1 inch

If a 100-lb weight causes a spring

to compress 1 inch, the spring has

a spring rate of 100 lb

A constant-rate spring continues

to compress at the same rate

throughout its complete range of

deflection

If a constant-rate spring will

compress one inch under a

100-pound load, it will compress two

inches under a 200-pound load.

Many automotive suspension springs, both coil and leaf, compress at

a variable rate, becoming stiffer, exerting more force the further they compress.

Variable-rate springs offer a soft, comfortable ride under normal

circumstances but will not bottom out as quickly when adverse road conditions compress them further.

See Figure 85–12.

CAUTION: The use of spacers between the coils of a coil spring is not

recommended because the force exerted by the spacers on the springs can cause spring breakage When a spacer is installed between coils, the

number of coils is reduced and springs become stiffer Force exerted on the coil spring at contact points of the spacer can cause the spring to break.

CAUTION: The use of spacers between the coils of a coil spring is not

recommended because the force exerted by the spacers on the springs can cause spring breakage When a spacer is installed between coils, the

number of coils is reduced and springs become stiffer Force exerted on the coil spring at contact points of the spacer can cause the spring to break.

Figure 85–12 Variable-rate springs come in a

variety of shapes and compress more slowly

as weight is applied.

Before a spring is installed on a vehicle or any load is placed on

it, it is at its uncompressed length, or free length Once installed,

the weight of the corner of the vehicle resting on the spring is

called its static load.

The static load constantly compresses the spring

The uncompressed length and spring rate must be such that the spring has room to compress and keep the

vehicle at the correct ride

height after the static load

is applied.

See Figure 85–13.

Note each of the four coil springs used on this vehicle is unique

The higher spring rate on the left is used to help support the weight

of the driver Because each spring is designed for each location on the vehicle, they should be marked if removed during service.

Figure 85–13 Two springs, each with a different spring rate and length, can provide the same ride height even though the higher-rate spring will give a stiffer ride.

Does the Spring Rate Change as the Vehicle Gets Older?

No, the spring rate of a spring does not change, but the spring load can

change due to fatigue The spring rate is the amount of force it takes to

compress the spring 1 inch The spring load is the amount of weight that a spring can support at any given compressed height When a spring

fatigues, the spring’s load capacity decreases and the vehicle will sag.

See the chart on Page 1056 of your textbook.

Spring Frequency is the speed at which a spring oscillates, or

bounces, after released from compression or extension.

Figure 85–14 Stiffer springs bounce at a higher frequency than softer springs.

Frequency is typically measured

in cycles per second (CPS) or hertz (Hz)

There is a direct correlation between spring rate & frequency.

The higher the spring rate, the

higher the spring frequency

Stiffer springs bounce at a higher

frequency, while softer springs bounce more slowly.

Wheel Rate Depending on the suspension design, springs are

installed a certain distance away from the wheel, which determines

ratio of wheel travel to spring travel, or wheel rate

If a coil spring is mounted on the midpoint of a control arm, or

halfway between the center of the wheel and the arm pivot points, it compresses approximately 1 inch when the wheel travels vertically

2 inches.

On a strut-type suspension, the coil spring has a more direct ratio

because it is closer to the wheel When the wheel of a strut travels

vertically 2 inches, the spring compresses 2 inches A coil spring

used on a strut-type suspension is less than half the spring rate of a coil spring used on suspensions that use control arms.

See Figure 85–15

Figure 85–15a The wheel and arm

acts as a lever to compress the

spring The spring used on the top

picture must be stiffer than the

spring used on the strut-type

suspension shown on the bottom

because the length of the lever

arm is shorter.

Figure 85–15b The wheel and arm

acts as a lever to compress the

spring The spring used on the top

picture must be stiffer than the

spring used on the strut-type

suspension shown on the bottom

because the length of the lever

arm is shorter.

Spring Coatings All springs are painted or coated with epoxy to

help prevent breakage A scratch, nick, or pit caused by corrosion

can cause a stress riser that can lead to spring failure The service

technician should be careful not to remove any of the protective

coating Always use tools that will not scratch or nick the spring.

Coil Spring Mounting Coil springs are usually installed in a

spring pocket or spring seat Hard rubber or plastic cushions or

insulators are usually mounted between the coil spring and the

spring seat.

See Figure 85–16

Figure 85–16 The spring cushion helps isolate noise and vibration from being transferred to the passenger compartment (Courtesy of Cooper Automotive Company)

The purpose of these

insulators is to isolate

and dampen road noise

and vibration from the

vehicle body

The type of end on

the coil spring also varies

and determines the style

of the spring mount.

Don’t Cut Those Coil Springs! - Part 1

Chassis techs are often asked to lower a vehicle One method is to remove coil springs and cut off half or more coils While this will lower the vehicle,

this method is generally not recommended for the following reasons:

1 A coil spring could be damaged during the cutting-off procedure,

especially if a torch is used to do the cutting.

2 Springs get stiffer when shortened, resulting in a very harsh ride.

3 The amount the vehicle is lowered is less than the amount cut off

the spring This is because as the spring is shortened, it becomes

stiffer The stiffer spring will compress less than the original.

Instead of cutting springs to lower a vehicle, several preferable methods are available if the vehicle must be lowered:

1 Replacement spindles designed to raise the location of the wheel

spindle, which lower the body in relation to the ground Except for

ground clearance problems, this method is recommended by many

chassis techs They keep the same springs, shock absorbers, and

ride, lowering the vehicle without serious problems.

Don’t Cut Those Coil Springs! - Part 2

2 There are replacement springs designed specifically to lower that

model vehicle A change in shock absorbers may be necessary

because the shorter springs change the operating height of the stock

(original) shock absorbers.

Figure 85–17

The replacement coil spring (left)

designed to lower a vehicle is next

to the original taller spring (right).

LEAF SPRINGS

Leaf springs are constructed of one or more strips of long, narrow

spring steel These metal strips, called leaves, are assembled with

plastic or synthetic rubber insulators between the leaves, allowing

for freedom of movement during spring operation

Figure 85–18 A typical leaf spring used

on the rear of a pickup truck showing the

plastic insulator between the leaves,

which allows the spring to move without

creating wear or noise.

Figure 85–19 A typical leaf spring installation The longest leaf, called the main leaf, attaches to

The ends of the spring are rolled or looped to form eyes Rubber

bushings are installed in the eyes of the spring and act as noise

and vibration insulators

The leaves are held together by a center bolt, also called a centering

pin.

Figure 85–20 All multileaf springs use a center bolt to not only hold the leaves together but